(1) Field of the Invention
This invention relates generally to liquid crystal technology.
More specifically, this invention relates to liquid crystal apparatus in a polymer matrix capable of electronically controllable color variation.
(2) Description of the Prior Art
Liquid crystal displays which involve the incorporation of droplets of liquid crystals in a polymer matrix are known as polymer-dispersed liquid crystal (PDLC) displays or encapsulated liquid crystal (NCAP) displays.
This type of display (PDLC or NCAP) can be produced without the size restrictions which are encountered in conventional liquid crystal displays for reasons known to those skilled in the art.
A typical polymer-dispersed liquid crystal display (PDLC) is disclosed in U.S. Pat. No. 4,685,771 (1987) and in U.S. Pat. No. 4,688,900 (1987). The PDLC film comprises droplets of liquid crystal material dispersed in a flexible plastic film. The displays change from a scattering state to a transparent state by thermal, electrical, or magnetic means which change the orientation of the optic axes of the birefringent liquid crystal droplets in the polymer matrix. The index of refraction of the polymer matrix is matched to the ordinary index of refraction of the liquid crystals, so that, when the optic axes of the liquid crystals are oriented such that light is incident on the ordinary index of refraction of the liquid crystals, the display is transparent and, when light is incident on the extraordinary index of refraction of the liquid crystals, the incident light is scattered.
The encapsulated liquid crystal display (NCAP) is disclosed in U.S. Pat. No. 4,616,903 (1986). The NCAP display comprises specifically nematic liquid crystals encapsulated in a polymer medium. When interposed between two substrates with conductive layers thereon, the application of an electrical field thereacross changes the alignment of the director of the liquid crystals resulting in a variation from scattering to transparent state as with the PDLC display.
Where the PDLC and NCAP displays contain nematic liquid crystals, in the "off" state the indices of refraction of the liquid crystals are not aligned in any particular direction, and since some liquid crystal droplets have the extraordinary index of refraction parallel to the substrates, incident light will be scattered. When an electrical or magnetic field is applied transverse to the plane of the polymer film, the liquid crystal droplets' ordinary index of refraction is aligned parallel to the substrates, which results in a transmission of the incident light because the ordinary index of refraction of the liquid crystals matches the index of refraction of the polymer matrix.
A method has also been described whereby the extraordinary index of refraction of the liquid crystals is aligned during the curing process when making polymer matrix displays. In U.S. Pat. No. 4,688,900 (1987) a method is disclosed wherein an electrical field or a magnetic field of sufficient strength is applied across the plane of a PDLC film, straining the film in a particular direction after curing, to align the extraordinary index of refraction of the liquid crystals parallel to the substrates thus producing a film that polarizes one component of light. After the polymer matrix has cured, the alignment is permanent. Application of an electrical field or magnetic field normal to the plane of the PDLC film changes the direction of the extraordinary index of refraction normal to the incident light and reduces the polarization effects and scattering of light, thereby producing a controllable polarizer that varies from a scattering state to a transparent state. U.S. Pat. No. 4,944,576 (1990) describes the method of partially aligning one of the indices of refraction of the liquid crystal droplets while the polymer matrix is being cured.
In both the PDLC and NCAP displays, it is known from Drzaic et al, "High Brightness and Color Contrast Displays Constructed from Nematic Droplet Polymer Films Incorporating Pleochroic Dyes", SPIE, Vol. 1080, 1989 to add a positive order parameter dye to the liquid crystals, resulting in a display that exhibits a change from the color of the dye while in the scattering state, and then nearly no absorption in the "on" state when there is no scattering effect due to the rotation of the optic axes of the liquid crystal droplets. These displays produce only a change from a scattered color to a relatively transparent state, although there is still a problem with a small amount of color being apparent in the transparent state of the display, due to the imperfections related to the order parameter of dye always being less than one. This is illustrated in a commercial product application of a PDLC as a sun-roof in a Buick concept car (Popular Science, Vol. 241, No. 2 (1992), pp. 29-32), where the display is designed to change from color in the scattering state to clear in the transparent state, but in all states of transmission the sunroof still exhibits a tint of the color of the car. This can be a problem since it would be most desired to have absolutely no color appearance in the transmissive state. The solution to this problem heretofore has not been obvious to those skilled in the art.
In conventional liquid crystal displays, another variety of dye has been used in addition to the positive order parameter dye. In an article by Schadt, "Field-Induced Color Switching in Liquid Crystal Displays", J. Chem. Phys. Vol. 71(6), 15 Sep. 1979, pp. 2336-2344, a liquid crystal guest-host display comprising a liquid crystal host with positive dielectric anisotropy mixed with positive order parameter dyes and negative order parameter dyes, permits a change from one color to another color upon application of a voltage. This is achieved by the positive order parameter dyes changing from a colored to colorless state, while the negative order parameter dyes change from a colorless to colored state. This type of display is limited in size due to the force of gravity on the liquid crystals, and maintaining uniformity of cell thickness across a large display is very difficult.